Ceramics offer attractive physical and mechanical properties to designers of, for example, advanced gas turbine engines, fuel cells, and electronic devices. In many cases, optimum utilization of these materials requires that they be joined to a metallic structure or body. Brazing is usually the preferred way of achieving the bonding, but the differences in ceramic and metallic properties can cause problems. Ceramic materials have lower coefficients of thermal expansion than most metals or alloys. Significant differences in the thermal expansion coefficients between these materials can generate thermal stresses across the joint during the brazing process or in subsequent use. Moreover, standard brazing alloys do not wet and flow well on ceramic surfaces.
Techniques have been developed to overcome the problems noted above. Relatively soft metals like copper or nickel can be used to better accommodate thermal stresses. If hermetic seals between the metal and ceramic joint are not required, fiber metal pads can be used. To create a brazeable surface, a metallic coating can be applied to the ceramic by a process commonly referred to as metallizing. Direct brazing, on the other hand, allows the brazing alloys to bond directly to metals or alloys. This has been made possible by the development of activated brazing alloys, which can wet and bond to ceramic surfaces. Most activated alloys contain titanium as the active metal component, but other metals such as zirconium, yttrium, or niobium can be used. Typically, the active alloys are gold, silver, or copper based. The problem with these brazing alloys is that gold migrates into the metal bonded with the ceramic when it is exposed to high temperatures for long periods, whereas silver and copper break down when they are exposed to an oxidizing atmosphere, damaging the brazed joint.
Currently available ceramic-to-metal brazing techniques are usable for relative low temperature applications, from room temperature to about 400-575° F. Advanced applications now being contemplated will subject the joint to much higher temperatures for extended duration, and in highly oxygen rich or oxidizing atmospheres. For example, internal temperatures of solid oxide fuel cells (SOFCs) can reach 1840° F., and thousands of hours of service will be required. Gas turbine operation will require even higher temperatures for similar extended times.
Presently, there are no economical or practical techniques available to produce the ceramic-metal joints for high temperature, long duration applications. The present invention addresses these needs.